1. Field of the Invention
The present invention relates to a technique of suppressing a so-called auto doping when performing an epitaxial growth of a semiconductor substrate.
2. Description of the Background Art
There has conventionally been proposed the technique of forming a thin epitaxial layer such as of silicon in a thickness of about several hundreds of nanometers, on the surface of a semiconductor substrate such as a silicon substrate. In some cases, the impurity concentration of the silicon substrate is set to a relatively high value. For instance, when a P type silicon substrate is employed and a P type silicon is formed as an epitaxial layer, the boron concentration of the silicon substrate is set to about 1xc3x971018 cmxe2x88x923.
On the other hand, the art of epitaxial growth has presented the problem of contamination being called xe2x80x9cauto doping.xe2x80x9d This is such contamination that, during epitaxial growth or the heat treatment in the step of manufacturing a semiconductor device by using an epitaxial layer, the impurity contained in a semiconductor substrate diffuses outside the substrate, and the diffused impurity enters the epitaxial layer. For instance, epitaxial growth has suffered from the phenomenon that the impurity contained in a silicon substrate is introduced into the source gas of the epitaxial growth.
To suppress the above-mentioned contamination, there has been proposed a technique of covering a semiconductor substrate on the side opposite from an epitaxial layer to be disposed. FIG. 5 is a sectional view illustrating this conventional technique. An oxide film 3 is formed on the reverse surface of a semiconductor substrate 1, e.g., a surface not subjected to mirror finish, and an epitaxial layer 2 is formed on the surface opposite from the oxide film 3 (hereinafter referred to as xe2x80x9cmain surfacexe2x80x9d). With this structure, the oxide film 3 suppresses impurity diffusing from the reverse surface of the semiconductor substrate 1, which results in suppressing auto doping. The technique of providing the oxide film 3 on the reverse surface of the semiconductor substrate 1 is described in Japanese Patent Application Laid-Open No. 9-266145 (1997), which discloses a technique employing a spin on glass film and atmospheric pressure chemical vapor deposition.
When the oxide film 3 is disposed on the reverse surface of the semiconductor substrate 1, however, the resulting semiconductor substrate 10 suffers from a noticeable warpage.
According to a first aspect of the invention, a semiconductor substrate comprises: first and second surfaces; and an oxide film apart from the first and second surfaces and extending throughout the semiconductor substrate.
According to a second aspect, the semiconductor substrate of the first aspect is characterized in that the distance between the oxide film and the second surface corresponds to a thickness on the order of 10xe2x88x923 of a thickness of the semiconductor substrate.
According to a third aspect, the semiconductor substrate of the first or second aspect is characterized in that the oxide film has a thickness of 400 to 1000 nm.
According to a fourth aspect, the semiconductor substrate according to any of the first to third aspects further comprises an epitaxial layer disposed on the first surface.
According to a fifth aspect, a semiconductor device comprises a semiconductor element disposed in the epitaxial layer of the semiconductor substrate according to the fourth aspect.
According to a sixth aspect, a method of manufacturing a semiconductor substrate comprises the steps of (a) providing a semiconductor substrate having first and second surfaces; and (b) forming an oxide film apart from the first and second surfaces and extending throughout the semiconductor substrate.
According to a seventh aspect, the method of the sixth aspect is characterized in that the step (b) includes the steps of: (b-1) introducing oxygen ion into the semiconductor substrate from the second surface; and (b-2) performing heat treatment after the step (b-1).
According to an eighth aspect, the method of the seventh aspect further includes the step (c), after the step (b), of forming an epitaxial layer on the first surface.
According to a ninth aspect, the method of the seventh aspect further includes the step (d) of making a semiconductor element by using the epitaxial layer.
With the semiconductor substrate of the first aspect or the method of manufacturing a semiconductor substrate of the sixth aspect, the auto doping to a semiconductor device and the warpage of the semiconductor substrate can be suppressed when a semiconductor element is formed on the first or second surface.
With the semiconductor substrate of the second aspect, less semiconductor is present between the oxide film and the second surface, and therefore the amount of auto doping is negligible.
With the semiconductor substrate of the third aspect, any warpage can be suppressed without any loss of the effect of suppressing auto doping.
With the semiconductor substrate of the fourth aspect, the impurity diffusion from the second surface is suppressed, thus reducing impurity to be introduced into the epitaxial layer.
With the semiconductor device of the fifth aspect, it is easy to obtain a desired characteristic because of the use of an epitaxial layer that is less subject to auto doping.
With the method of the seventh aspect, it is able to manufacture the semiconductor substrate of the first aspect.
With the method of the eighth aspect, since the impurity diffusion from the second surface is suppressed, the impurity introduction is reduced even when an epitaxial layer is formed in the step (c).
With the method of the ninth aspect, it is easy to obtain a semiconductor device with a desired characteristic because of the use of an epitaxial layer that is less subject to auto doping.
It is an object of the present invention to provide a technique of suppressing not only auto doping but also the warpage of a semiconductor substrate.